Pharmaceutical products and many biological materials are temperature sensitive, in that freezing may damage the materials and temperatures that are too high may otherwise spoil the materials. Thus, during shipment and storage of these types of materials, they must often be maintained within a particular temperature range. One common range for such materials during shipment and/or storage is 2-8° C. Storage containers for such materials may be active or passive with respect to temperature control. For example, a refrigerator is an active storage container and typically includes a refrigeration unit to extract heat from inside the refrigerator to maintain the desired storage temperature. A styrofoam cooler is an example of a passive storage container that relies on thermally insulating materials to retard heat transfer through the container walls. Passive storage containers are sometimes used with ice or some other type of phase change material inside the container to keep the storage area at a stable temperature.
Both active and passive storage containers experience temperature fluctuations in their respective storage areas. The refrigeration unit of an active container may cycle off and on to maintain the desired temperature in the storage area, with the storage area temperature decreasing during on-cycles and increasing during off-cycles. Compressor-driven refrigeration systems also must be periodically defrosted, which may include heating the evaporator coils inside the refrigerator and an accompanying spike in temperature. The temperature inside some refrigerators can vary by as much as 8-10° C. or more over the course of a day, depending on the ambient temperature outside the refrigerator, the age of the refrigeration equipment, the number of door openings, defrost cycle conditions, the quality of the insulation of the refrigerator, and/or the location within the refrigerator.
In a passive storage container, the temperature in the storage area may vary significantly with location inside the container. For example, if the stored contents are in contact with ice at a temperature of 0° C., the stored contents can quickly fall below 2° C., while other regions of the storage area are warmer. In addition, after any phase change material has completely changed phase, the temperature change inside the container is limited only by the insulative properties of the container walls.